Work on methods and apparatus for the study of kidney epithelial cells grown as sheets on porous membranes continues to yield important results. The central element is the porous bottomed culture device (PBCD) in which the membrane is cemented to polycarbonate rings which provides electrical and chemical isolation between the solutions on each side of the membrane. With these PBCDs new knowledge of the development of epithelial is being obtained. The increased exchange at the basal side (blood side) leads to a vasopressin responses for A6 cells (from Xenopus laevis kidney) which is otherwise absent. Omission of all serum from the apical medium (urine side) results in a larger and more rapidly developing potential difference (PD) across the epithelia. Transparent collagen membranes on the PBCDs have allowed these devices to be used effectively for primary cultures such as for medulary thick ascending limb cells from rabbit kidney. The importance of Ca# activity in the control process of epithelial cells has resulted in a project aimed at improving the selectivity of Ca# electrodes. Improvement is particularly needed for electrodes with tip diameters less than 0.1 microns so that activities less than 10-to the minus 6 molal can be measured in epithelial cells.